Glass fibers are useful in a variety of technologies. For example, glass fibers are commonly used as reinforcements in polymer matrices to form glass fiber reinforced plastics or composites. Glass fibers have been used in the form of continuous or chopped filaments, strands, rovings, woven fabrics, nonwoven fabrics, meshes, and scrims to reinforce polymers. It is known in the art that glass fiber reinforced polymer composites possess higher mechanical properties compared to unreinforced polymer composites, provided that the reinforcement fiber surface is suitably modified by a sizing composition. Thus, better dimensional stability, tensile strength and modulus, flexural strength and modulus, impact resistance, and creep resistance may be achieved with glass fiber reinforced composites.
Chopped glass fibers are commonly used as reinforcement materials in reinforced composites. Conventionally, glass fibers are formed by attenuating streams of a molten glass material from a bushing or orifice. The glass fibers may be attenuated by a winder which collects gathered filaments into a package or by rollers which pull the fibers before they are collected and chopped. An aqueous sizing composition, or chemical treatment, is typically applied to the fibers after they are drawn from the bushing. After the fibers are treated with the aqueous sizing composition, they may be dried in a package or chopped strand form.
Chopped strand segments may be mixed with a polymeric resin and supplied to a compression- or injection-molding machine to be formed into glass fiber reinforced composites. Typically, the chopped strand segments are mixed with pellets of a thermoplastic polymer resin in an extruder. In one conventional method, polymer pellets are fed into a first port of a twin screw extruder and the chopped glass fibers are fed into a second port of the extruder with the melted polymer to form a fiber/resin mixture. Alternatively, the polymer pellets and chopped strand segments are dry mixed and fed together into a single screw extruder where the resin is melted, the integrity of the glass fiber strands is destroyed, and the fiber strands are dispersed throughout the molten resin to form a fiber/resin mixture. Next, the fiber/resin mixture is degassed and formed into pellets. The dry fiber strand/resin dispersion pellets are then fed to a molding machine and formed into molded composite articles that have a substantially homogeneous dispersion of glass fiber strands throughout the composite article.
Unfortunately, chopped glass fibers are often bulky and do not flow well in automated equipment. As a result, the chopped fiber strands may be compacted into rod-shaped bundles or pellets to improve their flowability and to enable the use of automated equipment, such as, for example, for transporting the pellets and mixing the pellets with the polymer resins. U.S. Pat. No. 5,578,535 to Hill et al. discloses glass fiber pellets that are from about 20 to 30% denser than the individual glass strands from which they are made, and approximately 5 to 15 times larger in diameter. These pellets are prepared by hydrating cut fiber strand segments to a hydration level sufficient to prevent separation of the fiber strand segments into individual filaments but insufficient to cause the fiber strand segments to agglomerate into a clump. The hydrated strand segments are then mixed for a period of time sufficient for the strand segments to form pellets. Suitable mixing methods include processes that keep the fibers moving over and around one another, such as by tumbling, agitating, blending, commingling, stirring and/or intermingling the fibers.
Sizing compositions, such as are used in reinforced composites, are well-known in the art and conventionally include a film forming polymeric or resinous component, a coupling agent, and a lubricant. A sizing composition is typically added to glass fibers to reduce interfilament abrasion and to make the glass fibers compatible with the polymeric matrices they are intended to reinforce. The sizing composition also ensures the integrity of the strands of glass fibers, e.g., the interconnection of the glass filaments that form the strand.
One fundamental problem associated with conventional sizing compositions used in reinforced composites is the aging of the polymer matrix composite under the action of hydrolysis, which results in a decrease in mechanical strength. For example, hydrolysis of polyamide composites reduces tensile strength, elongation at break, and Charpy un-notched impact strength. Hydrolysis is a particular problem in areas where there is a significant amount of water present, such as in water end caps of radiators for automobiles. Thus, there exists a need in the art for a sizing composition that confers improved hydrolysis resistance to glass reinforced composites under extreme hydrolysis conditions.